Fluorescent lamps are used as a backlight in liquid crystal displays (LCDs), as employed, for example, in computer screens. Similar backlights are also found in other types of displays in a wide range of applications, such as in motor vehicles, illuminated advertising panels and suchlike.
Cold cathode fluorescent tubes are generally employed in backlights for LCD screens. They have the advantage of generating a small amount of heat combined with a relatively long useful life and high efficiency. Moreover, the electrode structures are simple making it possible to produce very small cold cathode fluorescent tubes that can also be used in small liquid crystal displays.
A cold cathode fluorescent tube comprises a tube having a high-voltage terminal at a first end of the tube and a low-voltage terminal at the second end of the tube. The high-voltage terminal is supplied with a high-frequency AC voltage, a typical supply voltage having a frequency of approximately 50 to 100 kHz and a voltage amplitude of approximately 500 to 1000 V. The low-voltage terminal is generally connected to ground. However, it is also possible to connect the two cold cathode fluorescent tube terminals to AC voltages that are offset by 180° with respect to each other (non-inverted and inverted), a virtual ground being located at about the center of the tube. This is especially expedient for particularly long tubes.
A key criterion for LCDs is that they illuminate the entire display surface as uniformly as possible. Depending on the size of the screen, two to 16, or even more, cold cathode fluorescent tubes are used for the backlight. The lamps are arranged parallel to each other, vertically above one another and their light is distributed on a liquid crystal plate via a reflector and via a diffuser plate. To achieve the most uniform distribution of brightness that is possible, it is not only necessary for the individual lamps to glow with the same brightness, but each individual lamp in itself must also emit a uniformly bright light along its length. An uneven distribution of brightness between individual lamps due to manufacturing tolerances may occur and can be kept under control to a certain extent by selection during the manufacturing process.
Cold cathode fluorescent tubes in liquid crystal displays are supplied with a high-frequency AC voltage via an inverter, called a backlight inverter. A reflector directs the light emitted by the lamps onto a diffuser plate which guides and distributes the light onto a liquid crystal plate. The liquid crystal plate is generally inserted between two polarization plates. The entire arrangement is held in a frame. Cold cathode fluorescent tubes have negative incremental resistance. Until the starting voltage is reached, a current having only a very low current intensity flows through the lamp. The intensity of the current then rises rapidly and the voltage drops at the same time. In order to compensate the negative incremental resistance of the fluorescent tube, in the prior art the tube is frequently connected in series with a capacitor with the aim of generating an overall positive incremental resistance and stabilizing the circuit. This series capacitor is also referred to as “ballast” and is chosen to match the resistance of each fluorescent lamp. Due to manufacturing tolerances, the various fluorescent lamps that are connected in parallel in a lighting arrangement have different resistances, with the result that the effective lamp current of the fluorescent lamps vary, thus causing a non-uniform distribution of brightness. This problem is intensified in the situation where the brightness of the fluorescent lamp is reduced by analogue dimming.
U.S. Pat. No. 6,670,781 relates to the control circuit for cold cathode fluorescent tubes for LCDs and deals with the problem that, particularly in the case of analogue dimming, these lamps emit a non-uniform brightness and flicker. To solve this problem, U.S. Pat. No. 6,670,781 proposes a new control method for fluorescent lamps that uses a predetermined number of current pulses.
Other fluorescent lamps and particularly cold cathode fluorescent tubes for liquid crystal displays and associated control devices are described, for example, in U.S. Pat. Nos. 6,538,373 and 6,108,215.
Japanese Patent Application JP 2002 352974 A and the associated Patent Abstract describe a device for the control of cold cathode fluorescent tubes in a lighting arrangement having a plurality of such cold cathode fluorescent tubes, the current flowing through each cold cathode fluorescent tube being controlled. The current flowing through the tubes is measured and entered into a microcomputer via rectifier circuits. The microcomputer generates PWM control signals as a function of the measured tube currents to drive transistors for the control of the tube current. The cold cathode fluorescent tubes are connected in series with resistors that are bridged by the transistors. The series resistors generate additional losses in the circuit arrangement.
It is an object of the invention to provide a lighting arrangement having a plurality of fluorescent lamps, and particularly cold cathode fluorescent tubes, that emit a uniform brightness in both normal operation as well as over a wide dimming range.